Ferric chloride has been used for many years as a catalyst during conversion of petroleum into asphalt to catalyze oxidation of the petroleum molecules to higher-molecular-weight, higher-viscosity forms. Some catalyst typically remains unused at the end of the process, and Tomlinson et al, U.S. Pat. No. 3,332,830 teaches that this retained catalyst is useful to improve the fire-resistance of asphalt-impregnated sheeting made from the asphalt. In the presence of a flame the catalyst causes the asphalt to increase in viscosity and ultimately to char, thereby limiting the flow of asphalt that could contribute to spreading of the flame.
Despite its improved flame resistance, asphalt catalyzed with the retained ferric chloride has limited utility, because reactions are catalyzed in the asphalt prematurely. For example, in the presence of the heat of a manufacturing operation, the ferric chloride catalyzes reactions that thicken the asphalt and make it difficult to impregnate and coat.
British Pat. Nos. 1,300,937 and 1,457,999 do not mention ferric chloride, but they suggest the use of materials that presumably can react to form ferric chloride. Specifically, these patents suggest that compositions of asphalt, bitumen, or pitch be made flame-retardant through the combined use of a halogen donor and a finely divided material that consists of aluminum-iron silicate and/or ferric oxide. The ferric oxide is regarded in the patents as activating or reacting with the halogen donor to enhance the release of halogen or hydrogen halide when the composition is exposed to a flame. The halogen donor may be selected from chlorinated rubber and alkyl benzenes containing two or more chloride and/or bromine atoms as nuclear substituents. The compositions include, per 100 parts of bitumen, pitch, or asphalt, 1-50, preferably 4-10, parts of halogen donor, and 6-20, preferably 8-15, parts of ferric oxide.
The described teachings of the British patents could have only a limited utility, since halogen donors are typically much more expensive than asphalt, e.g., 20 times more expensive than asphalt. The use of 1-50 and preferably 4-10 parts of the halogen donor would greatly increase the cost of asphalt, to a point that it would be impractical for most uses of asphalt.